Rare-earth-doped yttria-gadolinia ceramic scintillators

ABSTRACT

Rare-earth-doped, polycrystalline yttria-gadolinia ceramic scintillators with high density, optical clarity, uniformity, cubic structure and which are useful in the detection of X-rays, include one or more of the oxides of rare-earth elements Eu, Nd, Yb, Dy, Tb, and Pr as activators. The oxides of elements Zr, Th, and Ta are included as transparency-promoting densifying agents. Any decrease in scintillator light output, due to the addition of transparency promoting additives, may be partially restored by the addition of either calcium oxide (CaO) or strontium oxide (SrO). Sintering, sintering combined with gas hot isostatic pressing, and hot pressing methods for preparing the ceramic scintillators are also described.

RELATED APPLICATIONS

This application is related to application Ser. Nos. 389,812, 389,814,389,816, 389,817, 389,818, 389,828, 389,829 and 389,830, all filed onthe same date and all assigned to the same assignee as the presentinvention.

BACKGROUND OF THE INVENTION

The present invention relates to rare-earth doped ceramic scintillatorsfor computerized tomography (CT) and other X-ray, gamma radiation, andnuclear radiation detecting applications. More specifically, theinvention relates to rare-earth-doped, polycrystalline, yttria/gadolinia(Y₂ O₃ /Gd₂ O₃) ceramic scintillators.

Computerized tomography scanners are medical diagnostic instruments inwhich the subject is exposed to a relatively planar beam or beams ofX-ray radiation, the intensity of which varies in direct relationship tothe energy absorption along a plurality of subject body paths. Bymeasuring the X-ray intensity (i.e., the X-ray absorption) along thesepaths from a plurality of different angles or views, an X-ray absorptioncoefficient can be computed for various areas in any plane of the bodythrough which the radiation passes. These areas typically compriseapproximately a square portion of about 1 mm×1 mm. The absorptioncoefficients are used to produce a display of, for example, the bodilyorgans intersected by the X-ray beam.

An integral and important part of the scanner is the X-ray detectorwhich receives the X-ray radiation which has been modulated by passagethrough the particular body under study. Generally, the X-ray detectorcontains a scintillator material which, when excited by the impingingX-ray radiation, emits optical wavelength radiation. In typical medicalor industrial applications, the optical output from the scintillatormaterial is made to impinge upon photoelectrically responsive materialsin order to produce electrical output signals, the amplitude of which isdirectly related to the intensity of the impinging X-ray radiation. Theelectrical signals are digitized for processing by digital computermeans which generates the absorption coefficients in a form suitable fordisplay on a cathode ray tube screen or other permanent media.

Due to the specific and demanding computerized tomography requirements,not all scintillator materials which emit optical radiation uponexcitation by X-ray or gamma ray radiation are suitable for CTapplications. Useful scintillators must be efficient converters of X-rayradiation into optical radiation in those regions of the electromagneticspectrum (visible and near visible) which are most efficiently detectedby photosensors such as photomultipliers or photodiodes. It is alsodesirable that the scintillator transmit the optical radiationefficiently, avoiding optical trapping, such that optical radiationoriginating deep in the scintillator body escapes for detection byexternally situated photodetectors. This is particularly important inmedical diagnostic applications where it is desirable that X-ray dosagebe as small as possible to minimize patient exposure, while maintainingadequate quantum detection efficiency and a high signal-to-noise ratio.

Among other desirable scintillator material properties are shortafterglow or persistence, low hysteresis, high X-ray stopping power, andspectral linearity. Afterglow is the tendency of the scintillator tocontinue emitting optical radiation for a time after termination ofX-ray excitation, resulting in blurring, with time, of theinformation-bearing signal. Short afterglow is also highly desirable inapplications requiring rapid sequential scanning such as, for example,in imaging moving bodily organs. Hysteresis is the scintillator materialproperty whereby the optical output varies for identical X-rayexcitation based on the radiation history of the scintillator. This isundesirable due to the requirement in CT for repeated precisemeasurements of optical output from each scintillator cell and where theoptical output must be substantially identical for identical X-rayradiation exposure impinging on the scintillator body. Typical detectingaccuracies are on the order of one part in one thousand for a number ofsuccessive measurements taken at relatively high rate. High X-raystopping power is desirable for efficient X-ray detection. X-rays notabsorbed by the scintillator escape detection. Spectral linearity isanother important scintillator material property because X-raysimpinging thereon have different frequencies. Scintillator response mustbe substantially uniform at all X-ray frequencies.

Among scintillator phosphors considered for CT use are monocrystallinematerials such as cesium iodide (CsI), bismuth germanate (Bi₄ Ge₃ O₁₂),cadmium tungstate (CdWO₄), and sodium iodide (NaI). Many of theaforementioned materials typically suffer from one or more deficienciessuch as excessive afterglow, low light output, cleavage, low mechanicalstrength, hysteresis, and high cost. Many monocrystalline scintillatorsare also subject to hygroscopic attack. Known polycrystallinescintillators are efficient and economical. However, due to theirpolycrystalline nature, such materials are not efficient lightpropagators and are subject to considerable optical trapping. Internallight paths are extremely long and tortuous, resulting in unacceptableattenuation of optical output.

Fabrication of monocrystalline scintillators for multicomponent powderconstituents is typically not economical and frequently impractical. Themulticomponent powder composition must be heated to a temperature aboveits melting point, and ingots of dimensions larger than those of eachdetector channel are grown from the melt. Considering the size of thebars required and the temperatures involved, the process is difficult inand of itself. In addition, some materials exhibit phase changes whilecooling, which would cause the crystals to crack when cooled after thegrowing process. Furthermore, single crystals tend to be susceptible tothe propagation of lattice defects along the crystal planes.

U.S. Pat. No. 4,242,221 issued to D. A. Cusano et al (assigned to thesame assignee as the present invention) describes methods forfabricating polycrystalline phosphorus into ceramic-like scintillatorbodies for use in CT.

The present invention provides improved ceramic scintillators composedof yttria-gadolinia and including a variety of rare earth activators forenhancing luminescent efficiency.

The terms "transparency" and "translucency", as used herein, describevarious degrees of optical clarity in the scintillator material.Typically, the inventive scintillator materials exhibit an opticalattenuation coefficient of less than 100 cm⁻¹, as measured by standardspectral transmittance tests (i.e., "narrow" angle transmission) on apolished scintillator material plate, at the luminescent wavelength ofthe respective ion. The most desirable scintillator materials have lowerattenuation coefficients and hence higher optical clarity(transparency).

SUMMARY OF THE INVENTION

The polycrystalline ceramic scintillators of the present inventioninclude between about 5 and 50 mole percent Gd₂ O₃, at least onerare-earth activator oxide selected from the group consisting of Eu₂ O₃,Yb₂ O₃, Dy₂ O₃, Tb₂ O₃, and Pr₂ O₃, at least one transparency promoterselected from the group consisting of ThO₂, ZrO₂, and Ta₂ O₅, and atleast one light output restorer selected from the group consisting ofCaO and SrO, the remainder being Y₂ O₃.

It is an object of the invention to provide polycrystalline,yttria-gadolinia ceramic scintillators having high X-ray stopping powerand high radiant efficiency.

It is another object of the invention to provide rare-earth-doped,polycrystalline yttria-gadolinia ceramic scintillators having highoptical clarity, high density, high uniformity, cubic structure andwhich are particularly useful in CT and other X-ray detectingapplications.

It is still another object of the invention to provide polycrystalline,yttria-gadolinia ceramic scintillators exhibiting low luminescentafterglow and low hysteresis.

BRIEF DESCRIPTION OF THE DRAWINGS

The features of the invention believed to be novel are set forth withparticularity in the appended claims. The invention itself, however,both as to its organization and method of operation, together withfurther objects and advantages thereof, may best be understood byreference to the following description taken in conjunction with theaccompanying drawings.

FIG. 1 depicts graphically the effect of increased thoria (ThO₂) contenton the light output and the light output restorative effects of CaO andSrO in a yttria-gadolinia ceramic scintillator containing 3 mole percentEu₂ O₃.

FIG. 2 is a graphical illustration of the dependence of the relativelight output on Eu₂ O₃ activator concentration in an inventivescintillator material containing 35 mole percent Gd₂ O₃, the remainderbeing Y₂ O₃.

FIG. 3a illustrates graphically the dependence of scintillatorefficiency on relative yttria-gadolinia content of an inventive ceramicscintillator containing 3 mole percent Eu₂ O₃ activator.

FIG. 3b is a graph illustrating 73 kev X-ray stopping power versusyttria-gadolinia compositional ratio of an inventive ceramicscintillator.

FIG. 4 is a graphical illustration of the effect of increased Yb₂ O₃concentrations on scintillator material afterglow.

FIG. 5 depicts graphically the relative light output of an inventivescintillator material with increased Yb₂ O₃ content.

FIG. 6 is a graphical illustratiion of the effect of increased SrOconcentration on scintillator material afterglow.

DETAILED DESCRIPTION OF THE INVENTION

U.S. Pat. No. 3,640,887, issued to R. C. Anderson and assigned to thesame assignee as the present invention, describes the manufacture oftransparent polycrystalline ceramic bodies. The bodies include theoxides of thorium, zirconium and hafnium and mixtures thereof withoxides of rare earth elements 58 through 71 of the Periodic Table. Thebodies may optionally include yttria. The average ionic radius of therare earth oxide with or without yttria must not exceed about 0.93 Å,and the difference in ionic sizes of the constituent oxides should notexceed 0.22 Å. The polycrystalline ceramics are disclosed as useful inhigh temperature applications and/or applications requiring lighttransmission. Exemplary applications include high temperaturemicroscopes, lamp envelopes, laser applications, and furnace windows.

The aforedescribed patent teaches that each polycrystalline ceramic bodyincludes between about 2 to 15 mole percent of thoria (ThO₂), zirconia(ZrO₂), hafnia (HfO₂), or some combination thereof to act as adensifying agent during sintering to promote transparency.

Translucent-to-transparent ceramic scintillator bodies composed of Y₂O₃, Gd₂ O₃, and one or more rare earth activator oxides, as describedand claimed in copending application Ser. No. 389,812, filedconcurrently herewith, by the same inventors as herein and assigned tothe same assignee as the present invention, can be produced without theaid of transparency promoters such as ThO₂, ZrO₂, Ta₂ O₅, and SrO.However, the transparency of some yttria-gadolinia ceramic scintillatorsis improved by the addition of transparency promoters. For example, atest pattern formed on one surface of a scintillator material, includinga transparency promoter, approximately 1.5 mm thick can be resolved onthe opposite surface of the scintillator material down to 7 line pairsper millimeter. Stated in another way, a series of 3 mil line widths canbe read through a 1.5 mm thick scintillator material sample. Without thetransparency promoter, a 1.5 mm thick sample will barely resolve oneline pair per millimeter, or stated differently, a series of 0.5 mmlines will be barely resolved by the 1.5 mm thick scintillator sample.It is apparent, therefore, that scintillator materials which includetransparency promoters have short optical escape paths from within anypart of the scintillator body where X-rays are absorbed. Suchscintillator materials are particularly desirable for CT use.

Applicants have found, however, that the inclusion of ThO₂, ZrO₂, orHfO₂ in the scintillator materials of the present invention in amountsspecified in the aforedescribed Anderson patent (about 2-15 molepercent) results in a material having greatly reduced light output whenexcited by high energy radiation such as X-rays. The resulting materialsare unsuitable for use in CT. It has also been found that Ta₂ O₅ alsoproduces a similar reduction in scintillator material light output.These tetravalent (4⁺) and pentavalent (5⁺) additive species have aninhibiting effect on the light output of the scintillator. It isimportant to note that light output refers to scintillation resultingfrom X-ray excitation. This is a significant distinction since someceramic bodies fluoresce under ultraviolet excitation, but do notscintillate upon X-ray excitation, for example.

Curve H, FIG. 1, illustrates relative light output (vertical axis) of apolycrystalline ceramic composed of about 58.7 mole percent Y₂ O₃, 38mole percent gadolinia (Gd₂ O₃), 3 mole percent Eu₂ O₃, 0.3 mole percentYb₂ O₃, with increasing thoria (ThO₂) in mole percent as shown in thehorizontal axis. As the quantity of ThO₂ is increased, the quantity ofY₂ O₃ is correspondingly decreased. The average ionic radius of theceramic constituents and the difference between ionic radii are asspecified in the Anderson patent. It is evident from Curve H, that thelight output for a material containing 2 mole percent ThO₂ (the minimumamount specified by Anderson) is only 5 percent of the light output forthe same material without thoria. In fact, the addition of as little as0.5 mole percent ThO₂, well below the lower limit specified in theAnderson patent, reduces the light output to a low value of about 18percent of that measured for the material without ThO₂. In anotherexample (not shown in FIG. 1), the relative light output of a ceramicbody containing 56.5 mole percent Y₂ O₃, 38 mole percent Gd₂ O₃, 3 molepercent Eu₂ O₃, 0.5 mole percent Yb₂ O₃, and 2 mole percent ZrO₂ is only4 percent of the light output of the same material without ZrO₂.

It has been discovered by the inventors herein that the light reducingeffect due to the addition of ThO₂, ZrO₂, and Ta₂ O₅ can be partiallyreversed by the addition of calcium oxide (CaO) or strontium oxide(SrO). The rejuvenating effect of CaO on the relative light outputdegraded by the addition of ThO₂ is illustrated by Curve K in FIG. 1.The relative light output of a scintillator material having 38.0 molepercent Gd₂ O₃, 3 mole percent Eu₂ O₃, 0.3 mole percent Yb₂ O₃, 0.5 molepercent ThO₂, and the remainder being Y₂ O₃ was observed to be 18percent of that of a material without ThO₂ (Curve H, FIG. 1). Curve Killustrates the light output restorative effect of CaO in a similarscintillator composition in which the molar ratio of CaO to ThO₂ is 2:1.Thus, the light output of a composition including 1.0 percent CaO and0.5 mole percent ThO₂ was restored to about 44 percent of that of amaterial without ThO₂. It is also evident from Curve K that the additionof 4 mole percent CaO to a composition containing 2 mole percent ThO₂increased light output from about 4 percent to about 13 percent.

A light output restorative effect has also been observed for SrO asillustrated by Curve J, FIG. 1, for the scintillator having theabove-described composition and a molar ratio of SrO to ThO₂ of 2:1. Forexample, the light output was observed to increase from 18 percent for amaterial containing 0.5 mole percent ThO₂, to about 32 percent for thesame material having in addition 1 mole percent SrO.

It has been found that SrO can also function as a transparency promoterin the yttria-gadolinia ceramic scintillator. The preferred molepercentage of SrO when used as a transparency promoter is between about0.1 and 2, and between about 0.5 and 3.0 when it is used as a lightoutput restorer. Preferred CaO content is between about 0.2 and 4 molepercent.

Transparency promoters ThO₂, ZrO₂, and Ta₂ O₅ are useful in quantitiesof up to 0.7 mole percent, 0.7 mole percent, and 0.5 mole percent,respectively. It will be noted on Curve H, FIG. 1, that if the molepercent of ThO₂ is kept below about 0.35 mole percent the light outputis not severely reduced. Hence, for some X-ray applications wheremaximum light output is not required, light output restoring additivesmay not be needed. Similarly if the mole percentages of ZrO₂ and Ta₂ O₅are maintained below 0.35 and 0.3, respectively, light output restoringadditives are unnecessary.

The oxides of rare-earth elements europium, neodymium, ytterbium,dysprosium, terbium, and praseodymium (Eu₂ O₃, Nd₂ O₃, Yb₂ O₃, Dy₂ O₃,Tb₂ O₃, and Pr₂ O₃, respectively) are added to the basicyttria-gadolinia phosphor system, including transparency promoters, asactivators to enhance scintillator efficiency. It is to be noted thatactivator efficiency is independent of the relative proportions of Y₂ O₃and Gd₂ O₃, or of the other scintillator material constituents.Generally, rare earth activator content may range between 0.02 and 12mole percent.

Yttria-gadolinia scintillators, containing Eu₂ O₃, exhibit excellentscintillating efficiency. Optimum concentration of Eu₂ O₃ is between 1and 6 mole percent. This is illustrated in FIG. 2 which shows that thehighest relative light output, indicated on the vertical axis, isobserved for Eu₂ O₃ concentrations of between about 1 and 6 molepercent, as indicated on the horizontal axis. The curve depicted in FIG.2 was obtained by varying Eu₂ O₃ content of a scintillator materialcontaining 25 mole percent Gd₂ O₃, the remainder being Y₂ O₃.

Neodymium oxide (Nd₂ O₃) is preferably added in quantities of betweenabout 0.05 and 1.5 mole percent. Most preferably, however, Nd₂ O₃ isadded in concentrations of between about 0.1 and 0.5 mole percent.Preferred terbium oxide (Tb₂ O₃) activator concentration is betweenabout 0.05 and 3 mole percent, while the preferred concentration ofdysprosium oxide (Dy₂ O₃) activator is between about 0.03 and 1.0 molepercent. The preferred range of Yb₂ O₃ as an activator is between about0.1 and 2 mole percent. The preferred mole percentage for Pr₂ O₃activator is between about 0.02 and 0.05.

Eu₂ O₃ is the preferred activator followed, in order of preference, byNd₂ O₃ and Dy₂ O₃.

FIG. 3a illustrates the dependence of scintillator efficiency, asmeasured by relative light output, on the compositional ratio of ayttria and gadolinia in a scintillator material containing 3 molepercent Eu₂ O₃. The relative mole percentages of Gd₂ O₃ and Y₂ O₃ areshown on the horizontal axis, while the relative light output is shownon the vertical axis. The dashed line at 50 mole percent Gd₂ O₃ and 50mole percent Y₂ O₃ indicates the beginning of a gradual crystallinephase transition in the scintillator material structure from the cubicphase to the monoclinic phase. It will be observed that high relativelight output is obtained from scintillator materials containing up toabout 50 mole percent Gd₂ O₃. Scintillator materials containing between50 and 65 mole percent Gd₂ O₃ exhibit modest relative light output, butare increasingly subject to grain boundary cracking and reduced relativelight output due to progressive transition from cubic to monocliniccrystalline phase.

The cubic crystalline phase is characterized by a high degree ofscintillator material structural symmetry. Materials having suchstructure are particularly desirable for CT applications. Scintillatormaterials having increasing amounts of monoclinic phase arecharacterized by lower relative light outputs and poor optical claritydue to grain boundary cracking and nonuniform crystalline structure.Materials having such noncubic structure exhibit appreciable lightscattering and reabsorption due to a longer effective relative lightpath length, thereby reducing the amount of light available fordetection by external photosensors.

In considering the usefulness of the scintillator material in CTapplications, the X-ray stopping power of the material must also beconsidered. FIG. 3b illustrates the dependence of 73 kev X-ray stoppinglength versus yttria-gadolinia compositional ratio for transparent andefficient scintillators. Stopping power is measured in terms of X-raystopping length, i.e., the distance an X-ray photon penetrates into thescintillator prior to its conversion to optical wavelength photons whichare detectable by photosensors. X-ray stopping length is primarilydependent on Gd₂ O₃ content and, as shown in FIG. 3b, increases theincreased Gd₂ O₃ concentrations. Generally, it is preferred to usebetween about 5 mole percent and 50 mole percent Gd₂ O₃. Materialscontaining less than about 5 mole percent Gd₂ O₃ exhibit low X-raystopping power for most practical design, while materials having morethan 50 mole percent are increasingly non-cubic and exhibit poor opticalclarity. A more preferred range of Gd₂ O₃ content is between 20 and 40mole percent. The most preferred range of Gd₂ O₃ concentration isbetween 30 mole percent and 40 mole percent, corresponding to an X-raystopping length of about 0.45 mm. For a 2 mm thick scintillator materialhaving an X-ray stopping length of 0.45 mm, approximately 99 percent ofX-ray photons entering the material are converted to optical wavelengthphotons.

Certain additives are useful in the yttria-gadolinia scintillator systemof the present invention to reduce undesirable scintillator materialluminescent afterglow, which may lead to undesirable distortion and thepresence of artifacts in reconstructed images. The luminescent afterglowphenomenon is classifiable into primary or fundamental afterglow andsecondary afterglow. Primary afterglow is of relatively short duration(up to approximately 3 milliseconds), while secondary afterglow may beseveral times to much more than several times the primary decay time.Fundamental luminescent afterglow of a phosphor is thought to beinextricably associated with the specific activator identity and theactivator local environment in the host matrix (in this caseyttria-gadolinia). The secondary, and most objectionable type ofafterglow, can be associated with more subtle changes in the activatorenvironment or simply with the presence of additional electron-hole"trapping" centers created by native defects and/or low level impuritiesat other sites in the host crystal. Both types of afterglow may bereduced by suitable purification or the addition of compensatingdopants. The added dopants used to reduce afterglow do so by forming"killer" centers which are believed to compete with the activatorcenters for electron-hole pairs that otherwise combine radiatively atthe activator centers.

Luminescent afterglow of rare-earth doped yttria-gadolinia ceramicscintillators of the present invention can be substantially reduced bytwo types of additives.

The addition of ytterbium oxide (Yb₂ O₃), itself a luminescent activatorin the yttria-gadolinia host if used alone as described heretofore,results in the reduction of undesirable secondary afterglow with onlyminor sacrifice of luminescent efficiency. If, as depicted in FIG. 4,the mole percentage of Yb₂ O₃ is increased from zero to about 2 molepercent, the primary or fundamental afterglow, τ, of the scintillatormaterial activated with 3 mole percent of Eu₂ O₃ is reduced from 1.1 to0.82 milliseconds. An increase in Yb₂ O₃ content from 0 to 2 molepercent is accompanied by the loss of nearly 50 percent of scintillatormaterial luminescent efficiency as graphically depicted in FIG. 5 inwhich relative light output is shown on the vertical axis, while the Yb₂O₃ concentration is shown on the horizontal axis.

Curves A, B, C, and D, depicted in FIG. 4, illustrate the fraction ofsecondary luminescent afterglow (vertical axis) remaining at timesgreater than 10 milliseconds (horizontal axis) following the cessationof X-ray excitation. For a scintillator material having 30 mole percentGd₂ O₃, 3 mole percent Eu₂ O₃, and 67 mole percent Y₂ O₃, but no Yb₂ O₃,it is evident from Curve A that at the end of 10 milliseconds followingX-ray turn-off, about three percent of the luminescence presentimmediately upon X-ray shut-off remains. Curves B, C, and D depictfractional afterglow for similar scintillator materials whichadditionally contain 0.2, 0.5, and 2 mole percent Yb₂ O₃, respectively,and correspondingly less Y₂ O₃. It is apparent that increasingquantities of Yb₂ O₃ reduce secondary afterglow. For example, at about10 milliseconds after X-ray turn-off, fractional afterglow for ascintillator material containing 2 mole percent Yb₂ O₃ (Curve D) isapproximately only 0.7 percent (7×10⁻³) of its value immediately upontermination of X-ray excitation as compared to about 3 percent (3×10⁻²)for a material without Yb₂ O₃ (Curve A). The addition of 0.3 molepercent Yb₂ O₃ to a scintillator composition made up of 66.7 molepercent Y₂ O₃, 30 mole percent Gd₂ O₃, and 3 mole percent Eu₂ O₃ resultsin an extremely useful CT scintillator material having a fast decaytime. Preferred concentration of Yb₂ O₃ for afterglow reduction isbetween about 0.15 and 0.7 mole percent.

Another additive dopant which is effective in reducing scintillatormaterial luminescent afterglow is strontium oxide (SrO). The addition ofSrO results primarily in the reduction of secondary afterglow withrelatively little sacrifice of luminescent efficiency. In theyttria-gadolinia scintillator system, the quantity of SrO generallyfound to be useful in reducing afterglow is between 0.1 and 2 molepercent. It is shown in FIG. 6 that an increase in the quantity of SrOfrom 0 to 2 mole percent has no appreciable effect on primary afterglow,τ, 1.08 milliseconds and 1.10 milliseconds, respectively. However, thereis appreciable effect on secondary afterglow as depicted by curves E andF. A scintillator material having 30 mole percent Gd₂ O₃, 2 mole percentEu₂ O₃, 68 mole percent Y₂ O₃, but no SrO (curve E) exhibits, at about150 milliseconds after X-ray turn-off, about 0.8 percent (8×10⁻³) of theluminescence present immediately after X-ray shut-off. Scintillatormaterials having the same composition (Curve F) but including 2 molepercent SrO (and 2 mole percent less Y₂ O₃) exhibit, after the sameelapsed time, only about 0.03 percent (3×10⁻⁴) fractional afterglow asindicated on the vertical axis in FIG. 6.

The aforedescribed yttria-gadolinia rare-earth-doped ceramicscintillator materials may be prepared by sintering, sintering plus gashot isostatic pressing, and hot pressing ceramic methods. The ceramicscintillator materials are preferably and most economically fabricatedby employing a sintering process.

A preliminary step in the fabrication of the ceramic scintillators, byany of the aforementioned methods, requires the preparation of asuitable powder containing the desired scintillator materialconstituents. In accordance with a first method for preparing such apowder, submicron-to-micron powders of yttria (Y₂ O₃), gadolinia (Gd₂O₃) having purities of, for example, 99.99 percent to 99.9999 percentare mixed with the desired rare earth activators in the form of oxides,oxalates, carbonates, or nitrates and mixtures thereof. The mixing ofthe selected constituents may be carried out in an agate mortar andpestle or in a ball mill using water, heptane, or an alcohol (such asethyl alcohol) as liquid vehicles. Dry milling may also be used for bothmixing and breakup of powder aggregates. If dry milling is employed, agrinding aid such as 1 to 5 weight percent of stearic acid or oleic acidshould be employed to prevent powder packing or sticking inside the ballmill. Transparency promoters such as SrO, Yb₂ O₃, Ta₂ O₅, ZrO₂ and ThO₂may also be added in the form of oxides, nitrates, carbonates, oroxalates before ball milling. If the various additives are nitrates,carbonates, or oxalates, a calcining step is required to obtain thecorresponding oxides prior to fabrication of the ceramic scintillator byany of the methods described hereinafter.

A second approach to obtaining the desired scintillator starting powderemploys a wet chemical oxalate method. In this method, the selectedmolar percentages of the nitrates of predetermined ones of Y, Gd, Eu,Nb, Yb, Dy, Tb, Pr, and Sr are dissolved in water and coprecipitated inoxalic acid to form the respective oxalates. The oxalate precipitationprocess involves the addition of the aqueous nitrate solution of thedesired scintillator material constituents to an oxalic acid solutionwhich is, for example, 80 percent saturated at room temperature. Theresulting coprecipitated oxalates are washed, neutralized, filtered, anddried in air at about 100° C. for approximately eight hours. Theoxalates are then calcined in air (thermally decomposed) atapproximately 700° C. to about 900° C. for a time ranging from one tofour hours, to form the corresponding oxides. Typically, heating for onehour at 800° C. is sufficient. Preferably, if either the hot pressing orthe sintering method is used to prepare the scintillator, the oxalatesand/or the resulting oxides may be milled by one of several methods suchas ball, colloid, or fluid energy milling to enhance optical clarity.Milling of the powder for between one-half hour and ten hours has beenfound to be sufficient. It should be noted, however, that typically theoptical clarity of the scintillator is improved by milling the oxalatesand/or oxides regardless of the preparation method. Zirconium andtantalum which do not form stable oxalates are added to the calcinedoxalates in the form of oxides or nitrates. Other additives introducedas oxides include CaO and ThO₂. Since these additives are alreadyoxides, they are introduced after the calcination step.

Following the preparation of the selected powder composition by one ofthe methods described above, in accord with the preparation ofscintillator materials by sintering, selected amounts of the powdercomposition are formed into powder compacts by either die pressing, ordie pressing followed by isostatic pressing to further increase greendensity. A die material which is inert with respect to the scintillatorconstituents is preferred to avoid undesired reactions andcontaminations. Suitable die materials include alumina, silicon carbide,and metals such as molybdenum, hardened steel, or nickel-based alloys.The powder compacts are formed by die pressing at pressures of betweenabout 3,000 psi and 15,000 psi. Alternatively, the die pressed powdercompacts may be isostatically pressed at between about 10,000 and 60,000psi to further increase powder compact green density. If any grindingsaids or compaction aids (lubricants, such as waxes) have been used, anoxidation treatment to remove all organic additives can be employedprior to sintering.

During the sintering phase, the compacts are heated in a hightemperature tungsten furnace, for example, in vacuum or a reducingatmosphere such as a wet hydrogen atmosphere (dew point of about 23° C.,for example) at a rate of between approximately 100° C. per hour to 700°C. per hour to the sintering temperature of between 1800° C. and 2100°C. The sintering temperature is then held from 1 hour to about 30 hoursto cause extensive densification and optical clarity development. Upontermination of the sintering step, the compacts are cooled from thesintering temperature to room temperature over a period of time rangingfrom about 2 to 10 hours.

Sintered ceramic scintillators may also be prepared by a heatingsequence which includes a hold at a temperature lower than the finalsintering temperature. Typically, the powder compact is heated at a rateof between 300° C./hr and 400° C./hr to a holding temperature of betweenabout 1600° C. and 1700° C. The holding period may range from 1 hour to20 hours, following which the temperature is raised to between about1800° C. and 2100° C. for final sintering for between 1 hour and 10hours. The increase from the holding temperature to the final sinteringtemperature is at a rate of between about 25° C./hr and 75° C./hr. Apreferred heating sequence comprises heating the powder compact to aholding temperature of 1660° C. in five hours, holding this temperaturefor ten hours, followed by heating to 1950° C. in 6 hours, and thensintering at 1950° C. for two hours. The preferred heating sequence wasused to prepare the aforedescribed ceramic scintillator materialsdiscussed in connection with FIG. 1.

Yttria/gadolinia ceramic scintillators for luminescent applications mayalso be prepared by a combination of processes involving sintering andgas hot isostatic pressing (GHIP). The starting oxide powdercompositions are prepared in accordance with one of the aforedescribedmethods. Preferably, the oxalate coprecipitation method is used. By wayof example and not limitation, one useful yttria-gadolinia scintillatorcomposition comprised 66.7 mole percent Y₂ O₃, 30 mole percent Gd₂ O₃, 3mole percent Eu₂ O₃, and 0.3 mole percent Yb₂ O₃. Another usefulcomposition comprised 49.7 mole percent Y₂ O₃, 45 mole percent Gd₂ O₃, 5mole percent Eu₂ O₃, and 0.3 mole percent Yb₂ O₃. In contrast to thepreviously described sintering process, which preferably requiresmilling of the oxalate and/or oxide powders to produce transparentceramics, the process of sintering combined with GHIP permits thefabrication of transparent ceramics from unmilled powders.

In the fabrication of yttria-gadolinia ceramic scintillators by thecombined processes of sintering and gas hot isostatic pressing,following the preparation of a powder having the desired composition,powder compacts are formed by cold pressing at pressures of between3,000 psi and 10,000 psi, followed by isostatic pressing at pressures ofbetween 15,000 psi and 60,000 psi. The pressed compacts are thenpresintered to 93 to 98 percent of their theoretical density attemperatures of about 1500° C. to 1700° C. for between 1 and 10 hours.The presintered compacts are then gas hot isostatically pressed withargon gas at pressures of 1,000 psi to 30,000 psi at temperaturesbetween 1500° C. and 1800° C. for 1 to 2 hours.

In accordance with an example of the preparation of a ceramicscintillator employing the sintering and GHIP technique, a powdercompact was formed by cold pressing approximately 20 grams of powder ina rectangular die at a pressure of approximately 4,000 psi. The samplewas then isostatically pressed at 30,000 psi to increase green densityto 49 percent of its theoretical value. The cold pressing of the samplewas followed by sintering in a wet hydrogen atmosphere (dew point 23°C.) for two hours at 1660° C. so that closed porosity is developed. Thedensity of the sintered sample, as measured by the water immersionmethod, at this stage in the fabrication process was determined to bebetween 93 and 98 percent of its theoretical value. In order to obtainadditional densification and optical transparency, the sample was gashot isostatically pressed in a carbon resistance furnace at 1750° C. forone hour at an argon pressure of 25,000 psi. During gas hot isostaticpressing, the temperature was increased to the final value of 1750° C.in a step-wise manner. The sample was initially heated in one hour to1400° C. and the temperature raised thence to 1750° C. in another hour.Following a holding period of one hour at 1750° C., the resultingceramic scintillator had a black appearance due to reduction in thereducing furnace atmosphere. The sample was rendered transparent tovisible light by suitable oxidation treatment such as heating in air ata temperature of 1200° C. for thirty-two hours. Comparison of thephysical dimensions of the sample before and after the GHIP treatmentindicates that the sample shrunk during the GHIP step, indicatingfurther densification. The finished ceramic exhibited a density ofgreater than 99.9 percent of theoretical value.

Transparent yttria-gadolinia ceramic scintillators may also be preparedby vacuum hot pressing a scintillator material powder prepared,preferably, by the aforedescribed wet oxalate coprecipitation process.In accordance with this method, a selected quantity of the calcinedoxalate powder is pressed in a graphite die with molybdenum foil used asspacers between the upper and lower graphite plungers. Alternatively, aboron nitride coated graphite die may be used. A pressure of about 1000psi to 1200 psi is applied at a temperature between about 600° C. and700° C. under a vacuum of less than 200 microns and maintained for aboutone hour. Thereafter, the pressure is increased to approximately between4000 psi and 10,000 psi and the temperature increased to between 1300°C. and 1600° C. The pressure is released after holding at the elevatedtemperature of between one-half to four hours and the sample furnacecooled to room temperature.

Ceramic scintillator samples prepared in accordance with thehot-pressing method may be discolored due to surface reaction with themolybdenum spacer. Additional discoloration may be due to oxygendeficiency in the furnace atmosphere during hot pressing. The ceramics,however, can be made optically clear by oxidation in air or anoxygen-containing atmosphere at a temperature of about 800° C. to 1200°C. for between one and twenty hours. Any residual discoloration may beremoved by conventional grinding and polishing techniques.

In a specific example of the preparation of a scintillator material bythe vacuum hot-pressing method, 10 grams of a scintillator oxidematerial were obtained from the aforedescribed oxalate coprecipitationprocess by calcination of the oxalates at 800° C. for one hour in air.The oxides were initially hot pressed in a boron nitride coated graphitedie at 700° C. for one hour under a vacuum of about 20 microns and at apressure of 1200 psi. The temperature and pressure were then increasedto 1400° C. and 6,000 psi, respectively, under a vacuum of approximately100 microns. These conditions were maintained for two hours, followingwhich the pressure was released and the resulting scintillator materialfurnace cooled.

The scintillator material was gray to gray-black in color due to thereducing atmosphere created in the hot press. Light grinding of thescintillator surface and heating at 950° C. for four hours removedcarbon sticking to the scintillator material. The remainder of the darkcoloration was removed by additional oxidation at 1150° C. for two hoursin air. The resulting scintillator material was "light tan" in color,translucent to transparent in optical quality, and exhibited goodrelative light output upon excitation by X-rays.

From the foregoing, it will be appreciated that rare-earth-doped,polycrystalline yttria-gadolinia scintillators having high density,optical clarity uniformity, cubic structure and which are useful incomputerized tomography and other X-ray detecting applications have beendisclosed. The scintillators also exhibit high X-ray stopping power andhigh radiant efficiency. Due to inclusion of specific additives,scintillator luminescent afterglow and hysteresis is minimized.

While certain preferred features of the invention have been shown by wayof illustration, many modifications and changes will occur to thoseskilled in the art. It is, therefore, to be understood that the appendedclaims are intended to cover all such modifications and changes as fallwithin the true spirit of the invention.

The invention claimed is:
 1. A polycrystalline ceramic scintillatorcomprising a composition consisting essentially of between about 5 and50 mole percent Gd₂ O₃, between about 0.02 and 12 mole percent of atleast one rare earth activator oxide selected from the group consistingof Eu₂ O₃, Nd₂ O₃, Yb₂ O₃, Dy₂ O₃, Tb₂ O₃, and Pr₂ O₃, at least onetransparency promoter selected from the group consisting of ThO₂ in anamount up to 0.7 mole percent, ZrO₂, in an amount up to 0.7 mole percentand Ta₂ O₅ in an amount up to 0.5 mole percent, said transparencypromoter being present in amount sufficient to improve the transparencyof the ceramic scintillator, and at least one light output restorerselected from the group consisting of CaO and SrO, in an amountsufficient to effect a higher light output than said ceramicscintillator absent said restorer the remainder being Y₂ O₃.
 2. Thepolycrystalline ceramic scintillator of claim 1 wherein saidtransparency promoter is ThO₂ present in an amount of between about 0.2and 0.7 mole percent.
 3. The polycrystalline ceramic scintillator ofclaim 1 wherein said transparency promoter is ZrO₂ present in an amountof between about 0.2 and 0.7 mole percent.
 4. The polycrystallineceramic scintillator of claim 1 wherein said transparency promoter Ta₂O₅ present in amount of between about 0.1 and 0.5 mole percent.
 5. Thepolycrystalline ceramic scintillator of claim 1 wherein Gd₂ O₃ ispresent in an amount of between about 20 and 40 mole percent.
 6. Thepolycrystalline ceramic scintillator of claim 1 wherein said lightoutput restorer is CaO present in an amount of between about 0.2 and 4mole percent.
 7. The polycrystalline ceramic scintillator of claim 1, 2,3, 4, 5, or 6 wherein the rare earth activator is Eu₂ O₃ present in anamount of between about 1 and 6 mole percent.
 8. The polycrystallineceramic scintillator of claim 1, 2, 3, 4, 5, or 6 wherein the rare earthactivator is Nd₂ O₃ present in an amount of between about 0.05 and 1.5mole percent.
 9. The polycrystalline ceramic scintillator of claim 1, 2,3, 4, 5, or 6 wherein the rare earth activator is Nd₂ O₃ present in anamount of between about 0.1 and 0.5 mole percent.
 10. Thepolycrystalline ceramic scintillator of claim 1, 2, 3, 4, 5, or 6wherein the rare earth activator is Tb₂ O₃ present in an amount ofbetween about 0.05 and 3 mole percent.
 11. The polycrystalline ceramicscintillator of claim 1, 2, 3, 4, 5, or 6 wherein the rare earthactivator is Yb₂ O₃ present in an amount of between about 0.1 and 2 molepercent.
 12. The polycrystalline ceramic scintillator of claim 1, 2, 3,4, 5, or 6 wherein the rare earth activator is Dy₂ O₃ present in anamount of between about 0.03 and 1 mole percent.
 13. The polycrystallineceramic scintillator of claim 1, 2, 3, 4, 5, or 6 wherein the rare earthactivator is Pr₂ O₃ present in an amount of between about 0.02 and 0.05mole percent.
 14. The polycrystalline ceramic scintillator of claim 1wherein said light output restorer is SrO present in an amount ofbetween about 0.1 and 3 mole percent.
 15. The polycrystalline ceramicscintillator of claim 14 wherein the rare earth activator is Eu₂ O₃present in an amount of between about 1 and 6 mole percent.
 16. Thepolycrystalline ceramic scintillator of claim 14 wherein the rare earthactivator is Nd₂ O₃ present in an amount of between about 0.05 and 1.5mole percent.
 17. The polycrystalline ceramic scintillator of claim 14wherein the rare earth activator is Nd₂ O₃ present in an amount ofbetween about 0.1 and 0.5 mole percent.
 18. The polycrystalline ceramicscintillator of claim 14 wherein the rare earth activator is Tb₂ O₃present in an amount of between about 0.05 and 3 mole percent.
 19. Thepolycrystalline ceramic scintillator of claim 14 wherein the rare earthactivator is Yb₂ O₃ present in an amount of between about 0.1 and 2 molepercent.
 20. The polycrystalline ceramic scintillator of claim 14wherein the rare earth activator is Dy₂ O₃ present in an amount ofbetween about 0.03 and 1 mole percent.
 21. The polycrystalline ceramicscintillator of claim 14 wherein the rare earth activator is Pr₂ O₃present in an amount of between about 0.02 and 0.05 mole percent.
 22. Apolycrystalline ceramic scintillator comprising a composition consistingessentially of between about 5 and 50 mole percent Gd₂ O₃, between about0.02 and 12 mole percent of at least one rare earth activator oxideselected from the group consisting of Eu₂ O₃, Nd₂ O₃, Yb₂ O₃, Dy₂ O₃,Tb₂ O₃, and Pr₂ O₃, and a transparency promoter selected from the groupconsisting of ThO₂ in an amount up to 0.35 mole percent, ZrO₂ in anamount up to 0.35 mole percent, and Ta₂ O₅ in an amount up to 0.3 molepercent, said transparency promoter being present in an amountsufficient to improve the transparency of the ceramic scintillator, theremainder being Y₂ O₃.
 23. The polycrystalline ceramic scintillator ofclaim 22 wherein the rare earth activator is Eu₂ O₃ present in an amountof between about 1 and 6 mole percent.
 24. The polycrystalline ceramicscintillator of claim 22 wherein the rare earth activator is Nd₂ O₃present in an amount of between about 0.05 and 1.5 mole percent.
 25. Thepolycrystalline ceramic scintillator of claim 22 wherein the rare earthactivator is Nd₂ O₃ present in an amount of between about 0.1 and 0.5mole percent.
 26. The polycrystalline ceramic scintillator of claim 22wherein the rare earth activator is Tb₂ O₃ present in an amount ofbetween about 0.05 and 3 mole percent.
 27. The polycrystalline ceramicscintillator of claim 22 wherein the rare earth activator is Yb₂ O₃present in an amount of between about 0.1 and 2 mole percent.
 28. Thepolycrystalline ceramic scintillator of claim 22 wherein the rare earthactivator is Pr₂ O₃ present in an amount of between about 0.02 and 0.05mole percent.
 29. The polycrystalline ceramic scintillator of claim 22wherein the rare earth activator is Dy₂ O₃ present in an amount ofbetween about 0.03 and 1 mole percent.
 30. The polycrystalline ceramicscintillator of claim 22, 23, 24, 25, 26, 27, 28, or 29 wherein the rareearth activator is Dy₂ O₃ present in an amount of between about 20 and40 mole percent.